1. Field of the Invention
The present invention relates to a control unit of a motor for an injection molding machine, in particular, to a control unit for controlling a servo motor used for a motor-driven injection molding machine or a hybrid molding machine.
2. Description of the Related Art
Drive methods of an injection molding machine are broadly classified into hydraulic and motor drive methods. While previously the hydraulic method had been mainly used, the motor drive method has now become more common. This is attributable to features of the motor drive method including a high rigidity of the power transmission mechanism, good ability to control position and speed of the movable parts, and a high energy conversion efficiency. However, the hydraulic method has a feature permitting easy and accurate force control of the driving section, that is unavailable in the motor drive method. A hybrid method has therefore been developed which combines the motor and the hydraulic drive methods.
FIG. 1 illustrates the configuration of a typical motor-driven injection molding machine. This motor-driven injection molding machine has an injection unit 10 and a mold clamping mechanism 20. The injection unit 10 comprises a hopper 11 for temporarily storing a raw resin material, a heating cylinder 12 for plasticizing and melting a resin fed from the hopper 11, and a screw 13 for metering the molten resin in the heating cylinder 12 and injecting the thus metered molten resin. The molten resin is injected into a cavity positioned between a fixed mold 21 and a movable mold 22.
The mold clamping mechanism 20 comprises the fixed mold 21, the movable mold 22, a fixed platen 23 and a movable platen 24, to which the fixed mold 21 and the movable mold 22 are attached, respectively, a toggle link 25 for moving the movable platen 24, and a plurality of tie bars 26 for guiding the movable platen 24.
The motor-driven injection molding machine further comprises a plurality of drive motors (servo motors). The plurality of drive motors includes an injection motor 14 to move the screw forward and backward; a metering motor 15 for rotating the screw 13; an injection unit moving motor 16 to move the entire injection unit 10 forward and backward; a mold opening/closing motor 27 for moving the movable platen 24; an ejecting motor 29 to move an eject pin 28 incorporated in the movable platen 24 forward and backward; and a mold thickness complying motor 30 to move the movable platen 24 and the toggle link 25 in accordance with the thickness of the fixed mold 21 and the movable mold 22.
The plurality of drive motors 14 to 16, 27, 29 and 30 are individually drive-controlled. For example, when all the drive motors are three-phase motors, as shown in FIG. 2, a servo controller 41 is connected via a three-phase inverter 40 to each of the drive motors 14 to 16, 27, 29 and 30. An encoder 42 for detecting rotation of the motors and two current sensors 43 for detecting the magnitude of drive current fed from the three-phase inverter 40 are attached to each of the drive motors 14 to 16, 27, 29 and 30. Detection signals from the encoder 42 and the current sensors 43 are fed back to the servo controller 41.
The servo controller 41 issues, under control of an upper control unit not shown, a control signal to the three-phase inverter 40 on the basis of the detection signals fed back from the encoder 42 and the current sensors 43. The three-phase inverter 40 generates signals (drive current) for three phases including U-phase, V-phase and W-phase in response to the control signal from the servo controller 41, and feeds the same to the three-phase motors. The three-phase motors thus rotate by an instructed amount of rotation at a timing instructed by the upper control unit.
In the motor-driven injection molding machine, as described above, the drive motors 14 to 16, 27, 29 and 30 are independently controlled by the corresponding servo controllers, and injection molding is thus carried out.
A hydraulic injection molding machine is characterized in that it is possible to achieve a larger transmission energy per unit time with a relatively small-sized apparatus. This is why there is a tendency toward adopting the hydraulic method for a large-scale (large-output) injection molding machine. There is, however, a demand for adopting the motor drive method or the hybrid method also for a large-capacity injection molding machine.
In order to adopt the motor drive method or the hybrid method for a large-capacity injection molding machine, it is necessary to provide a large-output motor. In order to control the large-output motor, it is necessary to increase the maximum dielectric strength or maximum current of the inverter. Along with this, the corresponding voltage must be increased, for example, from 200-V class to 400-V class, for the control system of the servo controllers or the like.
On the other hand, the maximum output torque of the motor required for the injection molding machine varies with the drive source. For example, there is a considerable difference between the maximum output torque that the mold opening/closing motor is required to have and the maximum output torque that the ejecting motor is required to have. Even within a molding cycle, the torque that a motor is required to have is not constant. A large torque is required in some cases, and only a small torque suffices in others. When adopting the motor drive method or the like for a large-capacity injection molding machine, therefore, it is necessary to provide motors, inverters and servo controllers in response to the maximum output torque and change in torque necessary for the individual driving sources. This presents a problem in that the individual motors cannot have a common control unit.
Japanese Unexamined Patent Application Publication No. 2000-41392 (hereinafter referred to as xe2x80x9cPublication 1xe2x80x9d) discloses a brushless DC motor comprising an inverter connected to two three-phase windings. Japanese Unexamined Patent Application Publication No. 7-298685 (hereinafter referred to as xe2x80x9cPublication 2xe2x80x9d) discloses an invention that can drive a six-phase induction motor by the use of two inverters. However, because the two inverters are connected to the same controller, it is necessary to change the configuration (software) of the controller in accordance with the purpose of use in the above-mentioned Publications 1 and 2. The inventions disclosed in Publications 1 and 2 suggest nothing about the following object of the present invention of a common control unit, and disclose or suggest nothing about means for achieving such an object.
Furthermore, in the conventional motor-driven type injection molding machine or hybrid molding machine, a single drive motor is connected to a single inverter, and a single servo controller is connected to this inverter. When the inverter fails and cannot be controlled by the servo controller, runaway of the drive motor may occur. If such a runaway occurs, for example, in the mold opening/closing motor 27, the movable mold 22 held by the movable platen 24 collides with the fixed mold 21 held by the fixed platen 23, thus leading to breakage of these molds.
In the control unit of the motor for the conventional injection molding machine, as described above, if an inverter fails and the motor is in a runaway state, a problem exists in that there are no means for stopping the motor. For example, the brushless DC motor disclosed in Publication 1 has an object to rotate the motor even when a problem occurs in an inverter circuit or the like. Publication 1 does not disclose or suggest anything about stopping a runaway motor.
Accordingly, it is an object of the present invention to make it possible to concurrently use various portions of a control unit for controlling drive motors used in an injection molding machine. In other words, the present invention has an object to achieve a control unit for controlling drive motors having a large maximum output by the use of circuits for a control unit for controlling drive motors having a small maximum output.
A control unit of a motor for an injection molding machine according to the present invention is for driving and controlling AC motors, each having a plurality of sets of three-phase windings, used for an injection molding machine.
The control unit according to a first aspect of the present invention has a plurality of three-phase inverters connected to the plurality of sets of three-phase windings, respectively, and a plurality of servo controllers connected to the plurality of three-phase inverters, respectively.
The control unit according to a second aspect of the present invention has a plurality of three-phase inverters connected to the plurality of sets of three-phase windings, respectively, and a plurality of servo controllers connected to the plurality of three-phase inverters, respectively. In addition, each of the plurality of three-phase inverters has a first self-diagnosing circuit. When abnormality is detected, the first self-diagnosing circuit issues a first abnormality signal to the servo controller connected to the three-phase inverter having the first self-diagnosing circuit having detected the abnormality. The servo controller having received the first abnormality signal transfers the first abnormality signal to the other servo controllers.